The present invention relates generally to systems for cleaning semiconductor wafers, and more specifically to a wafer cleaning method which sand blasts semiconductor layers and ohmic contact areas with particles of ice in an adjustable gas stream. Older methods for cleaning semiconductor wafers are:
1. scrubbing with brushes, generally with a fluid present;
2. use of high velocity sprays or streams of gases or other fluids;
3. chemical removal; and
4. abrasion with conventional abrasives, usually carried in a fluid.
All of these suffer from one of two problems. Either they do not remove sub-micrometer sized particles effectively, or they can add particulates themselves. Particulates of extremely small size bind very strongly to surfaces by electrostatic or capillary forces.
The task of providing on an improved method of cleaning semiconductor wafers is alleviated, to some extent, by the system disclosed in the following U.S. Patents, the disclosures of which are specifically incorporated herein by reference:
______________________________________ U.S. Pat. No. 4,744,834 issued to Hag; U.S. Pat. No. 4,300,581 issued to Thompson; U.S. Pat. No. 4,326,553 issued to Hall; U.S. Pat. No. 4,401,131 issued to Lawson U.S. Pat. No. 4,439,243 issued to Titus; U.S. Pat. No. 4,458,703 issued to Inue et al; U.S. Pat. No. 4,500,080 issued to Aigo; U.S. Pat. No. 4,585,517 issued to Stemple; U.S. Pat. No. 4,597,825 issued to Freeouf et al; and U.S. Pat. No. 4,038,786 issued to Fong. ______________________________________
Most of the above-cited references disclose systems for cleaning or etching substrate layers in semiconductor materials. These systems include the use of chemical strippers and abrasion systems.
The Lawson reference is interesting in that it combines ultrasonic energy with a semiconductor wafer cleaning fluid. The Hall reference cleans semiconductor wafers with a jet of cleaning fluid in a vibrating stream. While these systems are exemplary in the art, the use of chemical cleaning fluids has the intrinsic potential of creating additional contamination on the semiconductor.
The Titus and Hag references also use solvents to strip resist from a semiconductor substrate. The purpose of the Titus and Hag systems is somewhat different from those of Hall and Lawson (etching rather than cleaning substrate) yet they also have the potential of creating contamination on the semiconductor surfaces they treat.
The above-cited Fong reference is not involved with semiconductors at all, but is cited because it describes a new industrial cleaning system developed by the Lockheed Aircraft Corporation. The Fong reference discloses a system that cleans articles by sandblasting them with pellets of dry ice. The advantage of dry ice is that, unlike sand, the dry ice will disappear as a gas after sand blasting.
In view of the foregoing discussion, it is apparent that there remains a need to provide an improved means for cleaning semicondutor wafers without creating additional contamination on them. The present invention is intended to satisfy that need.